Railway train interval detection system



May 30', 1950 Filed- Sept. 23, 1947 P. M. BRANNEN 2,509,331

RAILWAY TRAIN INTERVAL DETECTION SYSTEM 2 Sheets-Sheet 1 71min 5 TrainA.

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,HTIORNEY MN Q r w S R Patented May 30, 1950 RAILWAY TRAIN INTERVALDETECTION SYSTEM Paul M. Brannen, Duquesne, Pa., assignor to The UnionSwitch & Signal Company, Swissvale, Pa., a corporation of PennsylvaniaApplication September 23, 1947, Serial No. 775,588

4 Claims. 1

My invention relates to distance measurin apparatus, and particularly toimproved means for measuring the distance between two railway trains.

The usual method of railway operation which provides safety for trainsproceeding in the same direction on the same track involvesestablishment of a space interval between the train in advance and thefollowing train. Among the means for establishing this space intervalare the well-known block signaling systems, both manual and automatic,cab signaling systems, etc.

In these systems, a railway is divided into a number of sections orblocks, which may vary in length from a few hundred feet to severalmiles. Information is supplied to the engineman of the train by waysideor cab signals as to whether the adjacent section in advance is or isnot occupied, or if a section some distance in advance is occupied.However, these systems do not provide a continuous measurement of theactual distance between trains, and the train in advance may have justentered a section, or may be leaving at the far end of a section, or maybe stopped anywhere within the section.

It is an object of my invention to provide improved means for measuringthe distance between two trains on a stretch of railway track.

Another object of my invention is to provideimproved means forcontinuously and reliably determining and indicating the distancebetween two trains proceeding in the same direction on a stretch ofrailway track.

A further object of my invention is to provide improved means fordetermining and indicating the distance between two trains proceeding inthe same direction on a stretch of railway track,

which means will promptly indicate improper operation.

Another object of my invention is to provide improved means fordetermining and indicating the distance between two trains proceeding inthe same direction on a stretch of railway track, which means compriseschecking means which normally operates to check the integrity of theapparatus, and which becomes inoperative during the time that theapparatus is measuring and indicating the distance between two trains.

Another object of my invention is to provide improved means of the typedescribed including means for continuously checking the proper operationof the apparatus which checking means is rendered inactive when twotrains come within the maximum range of the equipment. so that thechecking of the equipment does not inter- 2 fere with the measurement ofthe distance between the two trains.

In practicing my invention, I provide on the head or front end of eachtrain a first high frequency radio transmitter which is arranged totransmit radio energy at a first carrier frequency, hereinafter referredto as the interrogator frequency, from a first transmitting antennawhich is constructed and arranged to radiate substantially all of theenergy in a forward direction, 1. e., directive in a forward direction.Also on the head end of each train I provide a first high frequencyradio receiver which is arranged to receive radio energy of a secondcarrier frequency which is different from the first carrier frequency,and is hereinafter called the responder frequency. This receiverreceives energy from a first receiving antenna, which is forwardlydirective. The transmitter is controlled by a keyer which causes thetransmitter to continuously supply short pulses of radio energy of theinterrogator frequency to the associated transmitting antenna. Anindicator, which may be of the cathode-raytube type, is jointlycontrolled by the keyer and the first receiver in a manner to besubsequently explained.

.:.-On the rear end of each train, there is provided a second highfrequency radio receiver which receives energy from a second receivingantenna, directive to the rear. This second receiver is arranged torespond to radio energy at the interrogator frequency, and controls asec- 0nd radio transmitter which is arranged to transmit radio energy ofthe responder frequency from an antenna directive to the rear.

In operation, at the start of a measuring cycle the keyer causes thefirst transmitter to transmit a short pulse which is radiated in aforward direction from the head end of the train. When this pulse isreceived by the second receiver located on the rear end of theproceeding train, it is detected and amplified and is then applied tothe second transmitter to cause it to transmit a similar pulse at theresponder frequency which is radiated rearwardly from the preceedingtrain.

When this pulse is received by the first receiver,

is is amplified and demodulated, and thereafter supplied to theindicator.

At the time the keyer causes the first transmitter to supply an impulseof radio ener y. an impulse of energy is also supplied to the indicator.By any of several well-known means, the time interval between thesending of an impulse by the first transmitter and the reception of animpulse by the first receiver may be measured, and the information maythen be displayed by the indicator. It is apparent that. since thevelocity of propagation of radio waves is constant, the travel time ofan impulse is proportional to the distance covered, and consequently isproportional to the distance between the trains.

Accordingly, it will be seen that my invention provides for continuousmeasurement of the distance between the head end of one train and therear end of the preceding train.

The checking equipment comprises means for recurrently causing eachtransmitter to supply radio energy of the proper frequency to thereceiver associated with the transmitter, and means for detecting therecurrent operation of the receiver by this energy.

Other objects of my invention and features of novelty will be apparentfrom the following description taken in connection with the attacheddrawings.

1. shall describe two forms of distance measuring apparatus embodying myinvention, and shall then point out the novel features in claims.

In the drawings,

' Fig. 1 is a diagrammatic view of distance measuring apparatusembodying my invention.

Figs. 2 and 3 are illustrations of the face or screen of the indicatorunder different conditions, and Fig. 4 is a diagrammatic view of amodification of the equipment shown in Fig. 1 to provide checking of theequipment.

In the drawings similar reference characters refer to similar parts ineach of the two views.

Referring to Fig. 1, there is shown therein a stretch of railway tracki, over which two trains A and B are proceeding from right to left.

The head end of train A is equipped with. the

apparatus shown in a dotted rectangle and comprising a keyer It, atransmitter i2 with its associated antenna M, a receiver is with itsassociated antenna I8, a sawtooth generator 20, and a cathode ray tube22. The rear end of train B is equipped with the apparatus shown in adotted rectangle and comprising a receiver 24 with its associatedantenna 26, and a transmitter 28 with its associated antenna 30. Theantennas [4, I8, 26, and 30 may be of any conventional type havinghighly directive characteristics, such as, for example, an array ofelements with reflectors and directors. The antennas I4 and I8 arearranged to have a radiation pattern extending forwardly from the headend of train A, and the antennas 26 and 30 are arranged to have aradiation pattern extending rearwardly from the rear end of train B.

In operation, the transmitter l2, which is located on the head end oftrain A, is continuously pulse modulated by pulses supplied from thekeyer l0. As a result, short pulses of radio energy 3| of theinterrogator frequency Fl are radiated forwardly from the antenna I4.

Simultaneously with the delivery of a pulse to transmitter l2, the keyerl delivers a pulse to the sawtooth generator 20. The sawtooth gen--erator provides a deflection voltage for the horizontal deflectingplates 32 of the cathode-ray-' tube 22 and is arranged and constructedso that each time the keyer l0 supplies a pulse of energy to thesawtooth generator 20, the electron beam of the cathode-ray-tube 22 isdeflected horizontally across the face 38 of the tube from left to rightat a substantially constant rate and then returns to its originalposition in an extremely short time interval.

Accordingly, each time a pulse of radio energy is radiated from theantenna H, the electron beam of the cathode-ray-tube 22. starts from itsinitial position at the aft side of the tube, where it is positioned bycentering means,- not shown, and sweeps across the face of the tube fromleft to right at a predetermined speed. The sawtooth wave voltage thendrops suddenly to zero and the electron beam is returned to its originalposition in an extremely short interval of time.

The vertical deflecting plates 34 of the oathode-ray-tube 22 aresupplied with energy from the receiver l6. At this time, it is assumedthat no signals are being picked up by the receiver I8 and, accordingly,there is no deflecting voltage applied to the vertical deflecting plates34. As a result, the electron beam is not deflected vertically as ittravels across the tube, and, as shown in Fig. 3, the trace 38 displayedon the face 38 of the cathode-ray-tube is a straight line.

The equipment on the rear end of train B is constructed and a ranged sothat energy pulses 3! of interrogator frequency picked up by the antenna26 are supplied to the receiver 24 where the energy is demodulated andamplified, and is then supplied to the transmitter 28. The transmitter28 is constructed and arranged to supply radio energy of responderfrequency to the antenna 30 only upon the supply of energy from thereceiver 24. Accordingly, when no pulses of energy of interrogatorfrequency are picked up by the antenna 26, or when such pulses, ifpicked up by the antenna 26, are too weak to operate the receiver 24, itwill be seen that transmitter 28 remains inactive. When pulses of energyof ihterrogator frequency having sufficient signal strength are pickedup by antenna 26, the receiver 24 will cause the transmitter 28 toretransmit pulses of energy of responder frequency F2 I in accordancewith the pulses of energy of interrogator frequency which are received.

It will be apparent that the operating range of the equipment may beadjusted by varying the power output of the transmitters l2 and 28, andalso by adjusting the sensitivity of the receivers i6 and 24, so thatsignals of sufficient strength to operate the receivers l6 and 24 willbe received only when the trains are within a predetermined distancefrom each other, and so that the equipment will be inoperative when thedistance between the trains exceeds the predetermined distance.

It is now assumed that train A approaches within a predetermineddistance of train B, such as for example, 3000 yards. At this time thepulses 3| of energy of interrogator frequency F i are picked up byantenna 26 on the rear end equipment of train B and are of sufiicientstrength to operate receiver 24. These pulses are detected and amplifiedby the receiver 24 and are thereafter supplied to the transmitter 28. Aspreviously explained, the transmitter 28 is arranged and constructed sothat it transmits a pulse of energy of the responder frequency F2 inresponse to each impulse supplied to it from the receiver 24. The pulsesof energy of responder frequency F2 are radiated rearwardly from theantenna 30, and are picked up by the receiving antenna l8 on the headend of train A. After being demodulated and amplified by the receiverIS, the pulses are applied to the vertical deflecting plates 34 of thecathode-ray-tube 22, producing a vertical deflection or pip 40 on thetrace of the electron beam, as shown in Fig. 3.

Since the electron beam moves across the face of the tube at asubstantially constant velocity,

groomer and is started at the time a ulse of energy of interrogatorfrequency is transmitted, it will be seen that the thne at which thebeam is deflected vertically by a pulse of energy of responder frequencyis proportional to the time required for the pulse of energy ofinterrogator frequency to travel from train A to train B, plus the timefor a pulse of energy of responder frequency to travel from train B totrain A, neglecting the time re 'quired for the receiver 24 and thetransmitter 20. to respond to the signal and re-transmit the signal.

The velocity of radio waves in space is approximately 186,000 miles persecond, and is substantially constant, so that if the time required forthe signals to span a distance is known, that distance can be accuratelydetermined.

As shown in Figs. 2 and 3, a scale on which suitable distances aremarked is affixed to the face 38 of the cathode-ray-tube 22, andis'calibrated so that the point at which the vertical pip so appearsindicates thee between -train A and train B. This scale is corrected toallow for the delay between the reception of a pulse by the receiver 24and the re-transmission of the pulse by transmitter 28. The .pip" d9shown in Fig. 3 stands at the first large division on the scale, whichmay indicate, for example, a distance of 1000 yards.

. It will be apparent that, as the distance between the train A andtrain B changes, the "pip" 65 will move in accordance with the change.If the distance between the trains increases, the pip dd will move tothe right a proportional distance shown, is provided for the head endand for the rear end equipment, the positive and negative terminals ofthese sources are designated as x and 0, respectively. There is alsoprovided the usual source of plate, bias and filament voltage necessaryfor the operation of the electron tubes.

The coding devices CTA and-0TB are continuously connected to the lowvoltage source of energy and are equipped with contacts which arerecurrently operated at a selected rate, for exampie, 180 times perminute, and may be of the type shown and described in Letters Patent ofthe United States No. 1,913,826, issued to Herman G. Blosser on June 13,1933.

In operation, pulses from the keyer l0 modulate the transmitter i2 tothereby cause the along the trace 36, and if the distance between thetrains decreases, the pip" 60 will move to the left a proportionaldistance long the trace 38 toward the zero mark of the scale.

When train B approaches within a very short distance of train A, say,for example, 100 yards, the time required for the pulses of radio energyto travel between train A and train B becomes too short for theequipment to measure accurately,

but the reflected pulses still show as a pip" on or near the zero markof the scale, thereby indi cating that train A is extremely near trainB.

From the foregoing it will be seen that my invention provides means fordetecting the presence of a train ahead of another train, and provides acontinuous indication of the distance between the trains between certainpredetermined mammum and minimum distances.

In Fig. 4, there is shown a modification of the equipment shown in Fig.l and described above. The arrangement as shown in Fig. 4 provides meansfor continuously checking the operation of the equipment and provides awarning if the equipment is functioning improperly or is notfunctioning.

The equipment on train A and train B is similar to that shown in Fig. 1,but in addition there is provided on train A an auxiliary receiver atand its associated antenna 45, an auxiliary transmitter 48 with itsassociated antenna 50, a code foilowing relay DRA, a decodingtransformer D'lA, a code detecting relay CRA, an indicator lamp KA, awarning bell XA, and a coding device CPA. The equipment on train B issupplemented with an auxiliary transmitter 52 with its associatedantenna 54, an auxiliary receiver 56 with its associ ated antenna 58, acode following relay DRE, a decoding transformer DTC, a code detectingre.- lay CRB, an indicator lamp KB, a warning KB and coding device CTB.Y

.; Additionally, a low voltage source of energy, not.

transmitter J2 to continuously transmit short pulses 3| of radio energyof interrogator frequency Fl which are radiated in a forward directionfrom train A by the antenna I4. A ortion of the energy radiatedin thesepulses is picked up by the antenna 66, and is demodulated and amplifiedby the auxiliary receiver 44. During the closed periods of contact 60 ofcode transnutter CTA the amplified pulses are supplied to the auxiliarytransmitter 58 over the circuit including front contact 84 of relay CRAand back contact 08 of relay DRA and cause the transinitter 68 totransmit pulses of energy of responder frequency from its associatedantenna 88. The energy radiated from antenna 50 is "picked up by thereceiving antenna is and is demodulated and amplified by the receiver itand is then supplied to relay DRA. Accordingly, it is apparent thatduring the time that contact E8 of coding device CTA is closed, a pulseof energy of responder frequency is supplied through auxiliarytransmitter 68 to the receiver I6 each time that a pulse of interrogatorfrequency is radiated from the antenna M.

The code following relay DRA is supplied with energy from the output ofthe receiver i6 and is constructed and arranged so that its contactswill not be picked up by the usual noise" present in the receiver, butwill be picked up when and only when a signal of responder frequency ofa predet'ermined energy level is supplied to the receiver 1 6. when thecontact of coding device CTA is picked up, the recurring supply ofpulses from auxiliary transmitter 68 through the receiver it will causethe code following relay DRA to pick up its contacts. Although theduration of the dividual pulses is very short, the repetition fre- Aarmy is such that as long as contact 60 of code transmitter GPA isclosed substantially continuoils energy is supplied to the relay DRAwith the result that the contacts'of relay DRA pick x31) and remainpicked up throughout each closed period of the contact of codetransmitter CTA.

a, time interval, the coding device CTA o'pens its contact 60 therebyinterrupting the supply of pulses to the auxiliary transmitter 50. As aresult, the relay DRA is no longer supplied with energy from thereceiver i6 and the contacts of relay DRA release.

After a time interval, the contacts of coding device CTA again close,and pulses of energy of responder frequency are again supplied by theauxiliary transmitter to the receiver I6 in the of responder frequencyare being returned from the rear end of a preceeding train, andrecurrent groups of pulses of energy of responder frequency are suppliedlocally to the receiver II from the auxiliary transmitter 48, and therelay DRA picks up and releases its contacts in accordance with thegroups of pulses received by receiver IS.

The relay DRA has associated therewith a code detector relay CRA whichis energized by current supplied through the decoding transformer DTAwhen and only when the code following relay DRA follows coded energy, asexplained in Letters Patent the United States No. 2,237,788 to Frank H.Nicholson et a1.

At this time, therefore, the recurrent operation of thecontacts of relayDRA causes the contacts of relay CRA to pick up and remainpicked up.Front-contact e: of relay CRA establishes the circuit for supplyingenergy to the indies tion lamp KA, leach contact 6'2 interrupts thecircuit for supplying energy to the warning hell Xe, and front contactto in the circuit traced for. the auxiliary transmitter d8 remainsclosed.

The control grid $58 of cathode -ray tuhe 22 is supplied with a negativebias voltage from a source designated B(-) by a circuit includingcontact to of code transmitter CIA and a front contact Bil oi relay CRAin multiple with a hack contact of relay DEA, so that the trace #38 ofthe electron beam or the cathode-rey tuhe shown in Fig. 2 is recurrentlyflashed during the time that the coding device (ETA is operating andthis flashing of the trace on the face of the cathode-ray-tuhe indicatesthe operation of the coding device CTA. Intensity modulation of acathode-ray-tube in this manner is welidrnown in the art.

From the foregoing it will be seen that during the time in which thereis no train having rear end equipment within range, the equipmentlocated on the head end or a train operates to continuously transmitpulses of energy of the interrogator frequency. At recurrent intervals,pulses of responder frequency are transmitted to the receiver on thehead end of the train from an auxiliary transmitter in response to thereception of pulses of interrogator frequency detected by an auxiliaryreceiver. The recurrent response of a code following relay to thereceived pulses of energy of responder frequency causes a detector relayto be picked up, which relay controls ap propriate indication circuitsto indicate the proper functioning of the equipment.

The equipment provided for the rear end of each train, and shown in Fig.4, comprises a receiver 24 with its associated antenna 26, a transmitter28 with its associated antenna 30, an auxiliary transmitter 52 andantenna 54, an auxiliary receiver 56 and antenna 58, a code followingrelay DRE, :3, code detecting relay ORB with its associated decodingtransformer DTB, an indication light KB, a warning bell XB, and a codingdevice CI'B similar to the coding device CTA previously described.

It is assumed at this time that the equipment is operating normally andthat the rear end equipment shown in Fig. 4, is not within operatingrange of a train carrying the head end equipment previously described.The coding device CTB is continuously connected to the source of energy,and its contact I0 is recurrently opened and closed at a predeterminedrate. When contact of coding device CTB is closed, a circuit forsupplying energy to the auxiliary transmitter 52 is established. Thiscircuit is traced from the positive terminal of the high voltage sourcedesig nated B,(+), over contact I0 of coding device CTB, and over-frontcontact 12 of relay CR3 to the auxiliary transmitter 52. when relay D83is released its contact 14 also provides a circuit for supplying energyto transmitter 52 when the contact 10 of coding device CTB is closed.Accordingly at this time, the transmitter 52 transmits radio energy ofinterrogator frequency from its associated antenna 54 during the periodsin which contact 10 of coding device CTB is closed.

This energy is picked up by antenna. 26 and is detected and amplified bythe receiver 24, and is then supplied to the transmitter 28, whichthereupon transmits energy of responder frequency which is radiated fromantenna 30. This energy is picked up by the antenna 68 and sup- .pliedto the receiver 56 where it is demodulated,

amplified and supplied to the code following relay 333.. The codefollowing relay DRB is constructed and arranged so that it will pick upwhen and only when a signal of predetermined energy ievel is picked upby the antenna 58 and will not pick up its contacts as a result of the Iusual "noise in the output of the auxiliary receiver 55.

Accordingly, the contacts of relay DRE will pick up at this time.

When contact Ill of coding-device CTB moves to its open position, thesupply of energy to the auxiliary transmitter 52 is interrupted, and asa result radio energy of interrogator frequency is no longer supplied tothe receiver 24. Accordingly, the transmitter 28 ceases to supply energyof responder frequency to its antenna 30. With the interruption of thesupply of energy of responder frequency, the auxiliary" receiver 56 nolonger supplies to the relay DRB sufilcient energy to keep the relaycontacts picked up, and its contacts release.

After a short time interval, contact 70 of coding device C'I'B againcloses, and, as previously explained, the various components of theequipment operate with the result that the contacts of relayDRB againpick up.

From the foregoing it will be seen that the recurrentoperatlon ofcontact In of coding device CT'causes recurrent operation of the comtacts of 'relay DRB. By means of the decoding transformer DTB and therelay CRB, the recurrent operation of the contacts of relay DRB isdetected, as previously explained in connection with the equipmentprovided on the head end of the trains.

As long as the equipment is functioning properly, 'the recurrentoperation of the contacts of relay DRB supplies energy through thedecoding transformer DTB to the relay CRB andthe contacts of codedetecting relay CRIB are picked up.

Front contact 16 of relay CRB establishes the circuit. for supplyingenergy to the indication lamp KB, and back contact 16 of relay CRBinterrupts-the circuit for supplying energy to the warning bell XB.

It will be apparent from the foregoing description that if any part ofthe equipment becomes defective, the relay DRB will remain continuouslyenergized or deenergized. For example, if the receiver 24 should becomeinoperative, the transmitter 28 would not transmit pulses of responderfrequency and the auxiliary receiver 5' would not supply sufficientenergy to relay DRB to pick up its contacts, and its contacts wouldremain released. Likewise, if the transmitter 28 should become'deraneedso that it "continuously transmits energy regardless of whether or notenergy was supplied thereto from the receiver 24, the auxiliary receiver58 would continuously supply energy to the relay BBB, and its contactswould remain continuously picked up.

The cessation of operation of the contacts of relay DRB interrupts thesupply of energy to the relay CR3 and its contacts release. When contact1B of relay CRB releases, it interrupts the supply of energy to theindication lamp KB, and establishes the supply of energy to the warningbell XB, which indicates to the trainmen either that the equipmenthasfailed, or as hereinafter explained, that a following train is withinoperating range of the equipment so that the proper precautions may betaken to protect the train from'being overaken by a following train.

Additionally, it will be seen from the above description that theequipment on train A will function in a similar manner to indicate afailure of the equipment. For example, if the receiver it should becomeinoperative, the relay DRA would not receive sufilcient energy to pickup its contacts, and its contacts would remain continuously released,or, should the auxiliary transmitter 48 become defective, so that itcontinuously sup plies energy to its antenna 50, th receiver it wouldcontinuously supply energy to relay DRA, and the contacts of relay DRAwould be continuously picked up. As a result, the supply of energy torelay CRA through the decoding transformer DTA would be cut ofl, and thecontacts of relay CRA would release. When contact 62 of relay CRAreleases, it interrupts the supply of energy to the indication lamp KAand establishes the supply of energy to the warning bell XA, whichindicates to the trainmen either that the equipment has failed, or ashereinafter explained, that a preceding train is within operating rangeof the equipment and that the equipment will now operate to measure thedistance between the trains.

It is to be understood that the auxiliary transmitters t8 and 52 may beconstructed and arranged to have a power output which is weaker than theweakest signal which the receivers l6 and 2t will receive when thetrains are separated by the maximum distance through which it is desiredto detect and measure the interval between the trains. Additionally, theauxiliary receivers M and 56 may be constructed and arranged so thatthey will not respond unless the transmitters l2 and 28 are operatingat* their maximum power output, to thus insure that a drop in theefficiency of the equipment will be promptly detected.

I will now assume that the train A approaches within the operating rangeof the head end and rear end equipment which range may be, for example,3000 yards.

When this occurs the antenna 28 on the rear of train B picks up thepulses of energy of interrogator frequency transmitted by the head endequipment of train A in addition to the energy recurrently supplied byauxiliary transmitter 52. As pointed out above, the transmitter l2 onthe head end of train A transmits energy of the interrogator frequencysubstantially continuously. Accordingly, when this energy is received ontrain B, the substantially continuous supply of energy to the receiver24 causes the transmitter 28 to transmit energy of responder frequencysubstantially continuously. The auxiliary receiver 56, in

response to the substantially continuous supply of energy picked up byits antenna 58, supplies energy to relay DRB to keep the relay contactscontinuously picked up. The supply of energy I is to the relay CRB isinterrupted on the cessation of operation of the contacts of relay BBB,and relay CRB releases its contacts with the result that the indicationlamp m is extinguished and the warning bell X8 is sounded.

Additionally, when contact I2 of relay CRB releases it interrupts thecircuit previously traced for supplying energy to the auxiliarytransmitter 52 and as a result, the recurring transmission of energy ofthe interrogator frequency from the auxiliary transmitter is cut oil.Thereafter, the transmitter 28 transmits a pulse of energyof responderfrequency each time a pulse of interrogator frequency transmitted by theequipment on train A is received by the receiver 24. The repetition rateof these pulses is sumciently fast to provide a substantially continuousoutput from the auxiliary receiver 56 to relay DRB, with the result thatthe contacts of relay DRB remain picked up, and relay CRB remainsreleased as long as the equipments of trains A and B are withinoperating range of each other.

It will be seen from the foregoing that when the equipments on trains Aand B are within operating range of each other, the recurrent operationof the relay DRB ceases, and the relay CRB releases to interrupt therecurrent operation of the auxiliary transmitter 52. Thereafter, theequipment operates in such manner that a, pulse of responder frequencyis transmitted each time a pulse of interrogator frequency is received.

As previously explained, at the time train A advances close enough totrain B to be within the operating range of the equipment, the equipmenton the head end of train A is continuously transmitting pulses ofinterrogator frequency, a portion of which are received by the auxiliaryreceiver M and are supplied to the auxiliary transmitter 48 by thecircuit including contact 50 of coding device CTA. The response of thereceiver it to these pulses causes the contacts of relay DRA to operaterecurrently and the relay CRA therefore is energized.

When the pulses of responder frequency radiated from the rear end oftrain B are picked up by antenna it of train A, they cause energy to besubstantially continuously supplied from receiver it to relay DRA withthe result that the contacts of relay DRA pick up and remain picked up.Relay CRA thereupon releases, since energy is no longer supplied to therelay through the de coding transformer DTA. When contact 62 of relayCRA releases, it interrupts the circuit for the indication lamp KA andsounds the warning bell XA, to thereby call the attention of thetrainmen on train A.

When relay CRA releases, its front contact 66 interrupts the circuit forsupplying pulses from the auxiliary receiver 44 to the auxiliarytransmitter 48. However, the repetition rate of the pulses of. energy ofresponder frequency which are received from train B is such that therelay DRA is maintained picked up.

Also, when relay CRA releases, its front contact interrupts the biasingcircuit for the control grid 68 of the cathode-ray-tube 22 so that thetrace on the face of the cathode-ray-tube is not recurrently flashed atthis time.

At this time, when the keyer 10 supplies a pulse to operate thetransmitter H, a pulse is also supplied to the sawtooth wave generator20 to provide a deflection voltage for the deflecting plates 32 of thecathode-ray-tube. When a pulse of responder frequency is supplied to thereceiver it, it supplies a momentar demaybe provided a scale on the face38 of the cathode-ray-tube, graduated in convenient distances, and thedistance from the beginning of the trace to the pip is proportional tothe distance between the trains. Therefore, by means of theaforementioned scale, the direct distance from the head end of train Ato the rear end-of train B may be read directly from the scale.

Let it now be assumed that the train A decreases its speed, or train Bincreases its speed, so that the trains become separated by a distancegreater than the operating range of the equipments.

At this time the pulses of energy of interrogator frequency picked up bythe antenna 28 of the rear end equipment located on train B become tooweak to operate the receiver 24, and as a result, the supply of impulsesfrom the receiver 24 to the transmitter 28 is cut oil, and thetransmitter 28 ceases to transmit. Accordingly, pulses of responderfrequency are no longer picked u by the antenna 58, and the auxiliaryreceiver 56 ceases to supply energy to relay DRB.. When the relay DRBreleases its back contact H establishes the circuit includingcontact Illof code transmitter OTB for recurrentlv supplying energy to theauxiliarytransmitter 52. I

When the coding device C'IB closes its contact 10, energy ofinterrogator frequency is supplied from the auxiliary transmitter 52 tothe receiver 24, which'then causes transmitter 28, to transmit energy ofresponder freouency to the auxiliary receiver 56. The response ofauxiliary receiver 58 to the supply of energy of responder frequencycauses the relay DRB to pick up and its contact H interrupts the supplyof energy to the auxiliary transmitter with the result that thetransmission of energy outlined above is interrupted and the contacts ofrelay DRB release. When relay DRB releases, its contact '14 againestablishes the sunply of energy to auxiliary transmitter 52 andthecycle described above isrepeated. The rate of operation of the codingdevice CT is such that the relay DRB will complete several cycles ofthe"door bell action described above during one closed period of' thecontact of coding device CT.

The door bell" action of the contacts of relay DRB causes energy to besupplied through the decoding transformer D'IB to relay CRIB, whichthereupon picks up its contacts. when contact 12 of relay ORB picks up,it establishes the circuit previously traced for supplying energy to theauxiliary transmitter 62. and when contact 1 of relay'CRB picks up itestablishes the circuit for supplying energy to the indication lamp KBand interrupts the supply of energy to the Waming bell 'XB.

The rear end equipment of train 13 is now restored to its normalcondition as previously described. The apparatus on the head endof trainA also shows when the trains A and B become separated by a distancegreater than the operating range of the equipments. When the trainsbecome separated by this distance, the value of the energy of theimpulses of responder frequency supplied to the receiver It falls belowthe amount required to cause the receiver It to supply suflicient energyto maintain the contacts of relay DRA picked up, and as a result theconta ts 0! this relay release. When releases, it establishes thecircuit including the contact of code transmitter CTA for supplyingpulses of energy from, the auxiliary re-,

When contact 66 of relay DRA picks up, it' interrupts the circuit forconnecting the receiver H to the transmitter 48, and the consequentinterruption in the supply of pulses of responder frequency to thereceiver It causes the relay DRA to be deenergized and when contact 66of relay DRA releases, the cycle described above 1 is repeated.

'I'he recurrent pickup and release of the contents of relay DRA againcauses energy to be supplied to the relay CRA through decodingtransformer MA, and when contact 04 of relay CRA picks up it establishesthe circuit previously traced for connecting the auxiliary receiver 44to the auxiliary transmitter 48.

When contact 62 of relay CRA picks up, it interrupts the supply ofenergy to the warning bell XA and establishes the circuit for supplyingenergy to the indication lamp RA.

The equipment on the'head end of train A is now restored to its normalcondition as previous.- ly described. v Y

From the foregoing it will be seen that my invention provides means formeasuring and indi-- cating the distance between two trains proceedingin the same direction on a stretch of railway track, and additionallyprovides means for continuously checking the apparatus and promptlyindicating a failure which might result in the non-detection of a train.Additionally. my invention provides means for measuring and indi eatingthe distance between two trains proceeding in the same direction on astretch of railway claims without departing from the spirit scope of myinvention. Having thus described my invention, what I claim is: a

1. In a system for the measurement ofthe tance between a first and asecond vehicle, in combination, means located on said first vehicle forgenerating and radiating pulses of energy of a first frequency, meanslocated on said second vehicle for receiving said pulses of energy ofsaid first frequency and retransmitting pulses of energy of a secondfrequency, means located on said first vehicle for receiving said pulsesof energy of said second frequency, indicating means located on saidfirst vehicle and governed by said receivingmeans for visuallyindicating the time contact 66 of relay DRA can vehicle, auxiliarytransmitting means for transmitting energy to the receiving means or-the same vehicle, means for recurrently causing the operation of saidauxiliary transmitting means,

means for detecting the response of said receiving means to energyreceived from said auxiliary transmitting means, and indicating meanscontrolled by said detecting means.

2. In a system for measuring the distance between a first and a secondvehicle comprising a first principal radio transmitter located on saidfirst vehicle for transmitting pulses of radio energy of a firstfrequency to said second vehicle at a certain pulse rate, a firstprincipal radio receiver located onsaid second vehicle for receivingpulses of radio energy of said first frequency, a second principal radiotransmitter located on said second vehicle for transmitting pulses ofradio energy of a second frequency to said first vehicle, a secondprincipal radio receiver located on said first vehicle for receivingpulses of radio energy of said second frequency, and means located onsaid first vehicle for measuring the time interval between thetransmission of a pulse by said first principal transmitter and thereception of a pulse by said second principal receiver,-to therebyindicate the distance between said first and said second vehicle, thecombination with the foregoing apparatus of checking means located oneach vehicle for continuously and reliably checking the proper operationof the apparatus, said checking means comprising, a first auxiliaryradio transmitter and a first auxiliary radio receiver located on saidfirst vehicle, a second auxiliary radio transmitter and a secondauxiliary radio receiver located on said second vehicle, a coding devicelocated on each vehicle, said coding devices having contacts which arerecurrently operated at a rate substantially less than said pulse rate,a first code following relay responsive to energy supplied from saidecond principal receiver, a second code following relay responsive toenergy supplied from said second auxiliary receiver, a first codedetecting relay energized when and only when said first code followingrelay is following code, a second code detecting relay energized whenand only when said second code following relay is following code, awarning device on said first vehicle governed by said first codedetecting relay, a warning device on said second vehicle governed bysaid second code detecting relay, circuit means on said second vehiclefor causing said second auxiliary transmitter to operate each time saidcoding device associated therewith closes its contacts provided saidsecond code detecting relay is picked up or said second code followingrelay is released, and means for causing said first auxiliarytransmitterto operate in response to pulses of radio energy received by said firstauxiliary receiver when and only when the contacts of the associatedcoding device are closed provided said first code detecting relay ispicked up or said first code following relay is released.

3. In a system for measuring the distance between a first and a secondvehicle comprising a first principal radio transmitter located on said lergy-ofafirst frequency to said second vehicle at: a certain pulse rate,a first principal radio receiver located on said second vehicle forreceiving-pulses of radio energy of said first frequency, a secondprincipal radio transmitter located on said second vehicle fortransmitting pulses of radio energy of a second frequency to said firstvehicle, a second'principal radio receiver located on said first vehiclefor receiving pulses of radio energy of said second frequency, and meansincludinga cathode-ray-tube having a control electrode located on saidfirst vehicle for measuring the time interval between the transmissionof a pulse by said first principal transmitter and the reception of apulse by said second principal receiver, to thereby indicate thedistance between said first and said second vehicle, the combinatlonwith the foregoing apparatus of checking means located on each vehiclefor continuously and reliably checking the proper operation of theapparatus, said checking means comprising, a first auxiliary radiotransmitter and a first auxiliary radio receiver located on said firstvehicle, a second auxiliary radio transmitter and a second auxiliaryradio receiver located on said seccond vehicle, a coding device locatedon each vehicle, said coding devices having contacts which arerecurrently operated at a rate substantially less than said pulse rate,a first code following relay responsive to energy supplied from saidsecond principal receivena second code following relay responsive toenergy supplied from said second auxiliary receiver, a first codedetecting relay energized when and only when said first code followingrelay is following code, a second code detecting relay energized whenand only when said second code following relay is following code,

a warning device on said first vehicle governed I by said first codedetecting relay, a warning device on said second vehicle governed bysaid second code detecting relay, circuit means on said second vehiclefor causing said second auxiliary transmitter to operate each time saidcoding device associated therewith closes its contacts provided saidsecond code detecting relay is picked up or said second code followingrelay is released, means for causing said first auxiliary transmitter tooperate in response to pulses of radio energy received by said firstauxiliary receiver when and only when the contacts of the associatedcoding device are closed provided said first code detecting relay ispicked up or said first code following relay is released, and a circuitincluding a contact of the associated coding device for applying a biasvoltage to the control electrode of said cathode-ray-tube.

4. In a system for the measurement of the distance between a first and asecond vehicle, in combination, means located on said first vehicle forgenerating and radiating pulses of energy of v ceiving means forvisually indicating the time between the radiation of a pulse from saidfirst vehicle and the reception of a pulse from said second vehicle, andmeans located on said first vehicle for checking the operation of saidsystem, said checking means comprising auxiliary receiving meansresponsive to energy transmitted first vehicle for transmitting pulsesof radio n 7 from said vehicle, auxiliary transmitting means can:

' 15 for transmitting energy to the receiving means 01' the samevehicle, means for recurrently causmeans controlled by said detectingmeans.

PAUL M. REFERENCES crrEn The following references are of record in thefile of this patent:

1B UNITED STATES PATENTS N ber N ing the operation of said auxiliarytransmitting 3 fii ig ,g means, means for detecting the response of said3398.333 Tu'ot 1937 receiving means to energy received from said 53,134,718 Gum 1938 auxiliary transmitting means, and. indicating 3. 5

OTHER REFERENCES Pr 0! the IRE. vol. 30. No. 3, pp. 129 I, to 131 March

